1. Field of the Invention
The present invention relates to a resin-bonded magnet and its production. More particularly, it relates to a resin-bonded magnet improved in magnetic characteristics and heat stability, which comprises ferromagnetic alloy particles of a rare earth element system, and its production.
2. Description of the Related Art
It is difficult to make sintered magnets of Fe-R-B (wherein R is a rare earth element) alloys or intermetallic compounds in a cylinder shape magnetically anisotropic along the radial direction. The main reason for this is because the cylinder suffers a difference in expansion coefficient based on the anisotropy during the sintering process, which difference in expansion coefficient being more or less influenced by the degree of the magnetic anisotropy and the shape of the cylinder. In order to avoid said difficulty, the cylinder has thus been used in an isotropic state. This, however, involves a disadvantage in that while magnetic characteristics should intrinsically reach 20 to 30 MGOe in terms of maximum energy product, it lowers to about 5 MGOe along the radial direction of the cylinder. Further, the cylindrical magnet must be ground after sintering for incorporation into a permanent magnet motor in which a high dimensional accuracy is required. This apparently results in a poor yield of the magnet product. Furthermore, the sintered magnet is mechanically brittle so that a part of the magnet is liable to come off and fly apart. If this occurs at a space between the rotor and a stator of the motor or at a sliding portion, the motor would suffer a serious problem with respect to maintenance of its performance and reliability.
With the background above, it was proposed to apply a magnetically isotropic resin-bonded magnet of Fe-B-R produced by a melt quenching process to a permanent magnet motor (U.S. Pat. No. 4,689,163), and according to this proposal, it has been made possible to cope with various demands. However, such resin-bonded Fe-B-R magnet is still unsatisfactory in various magnetic characteristics. For instance, Fe.sub.83 Nd.sub.13 B.sub.4, as a typical example of said resin-bonded Fe-B-R magnet, shows the following magnetic characteristics irrespective of the magnet structure or shape or the magnetization direction: Br, 6.1 kG; bHc, 5.3 KOe; iHc, 15 KOe, (BH)max, 8 MGOe; temperature coefficient of Br, -0.19%/.degree. C.; temperature coefficient of iHc, -0.42%/.degree. C.; Curie temperature, 310.degree. C. For application to a permanent magnet motor, the decrease of the magnetization energy is desired. Also, the improvement of Br and heat, such as the irreversible demagnetizing factor, is desirable in view of the pronounced tendency toward high efficiency, miniaturization and resistance to surroundings of a permanent magnet motor.